Transducer



Dec. 9, 1969 D, L, FoLDs ET AL 3,483,504

TRANSDUCER Filed Aug 23. 1967 5 Sheets-Sheet 2 Ang/e 00m/d L, Fo/ds 56h56# Dov/'d H, Brown life/4M@ Henry L, Warner ww @hier INVENTORS 14k/d feder Dec. 9, 1969 D L, FOLDS ET AL TRANSDUCER 5 Sheets-Sheet 3 Filed Aug 23, 1967 Dona/d L. Folds Davia H. Brown Henry L' lxlgqfzfTroRs Dec. 9, 1969 D, v| FoLDs ET AL 3,483,504

TRANSDUCER Filed Aug 25, 196'? 5 Sheets-Sheet 4 522/596 1./ 5,4m my Wf- Fig IZ 100 Q d g Dang/d L, Fo/ds Qn a Q edv/d grown enr amer b y INVENToRs Dec. 9, 1969 D. L.. FoLDs ET AL TRANSDUCER 5 Sheets-Sheet Filed Aug 25, 1967 LD M Dona/0 L. Fo/ds Dav/'a H. Brown Henry L, War/7er mvENToRs MMA/gr /vEr nitedStates vPatent Owice 3,483,504 'I-'RANSDUCER Donald L. Folds, David H. Brown, and Henry L. Warner, Panama City, Fla., assignors to the United States of America as represented by the Secretary of the Navy Filed Aug. 2 3, 1967, Ser. No. 662,827 Int. Cl. H04b 13/00 ABSTRACT lon THE DISCLOSURE The present invention relates generally to electroacoustical transducers, and, in particular, is a reversible acoustic leus type of transducer which senses and focuses acoustical energy received from within a subaqueous medium in such manner as to resolve it into an image capable of being displayed as intelligible information by any compatible utilization and readout apparatus.

Focusing transducers are, of course, not new, and for some purposes appear to perform quite satisfactorily. However, in the past, electroacoustical transducers of the focusing type leave a great deal to be desired, inasmuch as the focal surface thereof upon which sensors must be placed exists at locations which limits the eld of View in proportion to-the size of the sensor array. Moreover, image :distortion usually occurs due to the diffraction effects inherently contained therein. Furthermore, prior art transducers of the solid thin lens type-that is, constructed of a solid refractive material--not only have a severely limited fields of view but also have aberrations caused by acoustic shear waves in said solid refractive material.

In the liquid lens electroacoustical type transducers of the prior art, a number of adverse characteristics have also 'been encountered. Although functional for some specific purposes, it has been found that they contain such primary aberrations as coma and astigmatism, whenever attempting to image objects that are not substantially located on the optical axis thereof.

The liquid filled acoustic lens transducer constituting the subject invention overcomes most of the disadvantages of theaforementioned prior art, in that spherical aberrations are reduced to substantially a negligible quantty, marginal lacoustical energy rays are practically eliminated for any desired reception angle, and directional response characteristics are capable of being optimized for given operational circumstances through design controls.

It is, therefore, an object of this invention to provide an improved electroacoustical reversible transducer.

Another object of this invention is to provide an improved method and means for acoustically imaging subaqueous target objects.

A further object of this invention is to provide an improved method and means for structurally tuning a liquid lens type underwater transducer having a wide iield of view to effect reduced aberrations therein.

A further object of this invention is to provide an improved liquid acoustic lens for an electroacoustic transducer.

3,483,504 Patented Dec. 9, 1969 Another object of this invention is to provide a new and improved acoustical energy refracting liquid.

Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings wherein:

FIG. l is a front quasi-pictorial view of the subject invention;

FIG. 2 is a rear quasi-pictorial view of the subject invention;

FIG. 3 is a sectional view taken at line 3-3 of FIG. 2;

FIG. 4 is a view of the energy converter array, as it is mounted as an assembly in the subject invention. In this view, parts are broken away from the housing portion thereof to facilitate disclosure simplification thereof;

FIG. 5 is a sectional View taken at line 5 5 of FIG. 2 illustrating the housing construction of the subject invention, showing, in particular, the mounting of the anechoic lining therein;

FIG. 6 is a detailed top view sample of the anechoic lining incorporated in the housing of the subject invention, the purpose of which is primarily to illustrate the waffle iron geometrical configuration thereof;

FIG. 7 is a functional schematic diagram depicting the theory of the general focusing system of the invention;

FIG. 8 is a schematic diagram illustrating representative acoustical ray paths which occur in the invention;

FIG. 9 is a diagrammatical representation of the theory of the path length as a function of incident angle which occurs in the subject invention;

FIG. l0 is a schematic representation of a cylindrical lens embodiment of the invention;

FIG. l1 is a graphical representation of relative response of the main lobe and first side lobe of the subject invention plotted as a function of d/)\, where d is the diameter thereof and A is the wavelength at which it is being operated;

FIG. 12 is a graphical representation of beamwidths of the subject invention, likewise plotted as a function of d/ FIG. 13 is a general focusing system diagram which illustrates the satisfying of a given focusing condition for an elliptical lens embodiment of the subject invention; and

FIG. 14 depicts a representative directivity-response pattern at two hundred kilocycles per second operation for a predetermined radial position of the subject transducer.

Referring now to FIGS. l through 5, there are shown various views of a preferred embodiment of the subject invention as having a watertight housing 21 having top and bottom aluminum plates 22 and 23 of predetermined thickness. A box-type aluminum rear wall 24, separating plates 22 and 23, extends approximately 120 around the rearward periphery of the transducer and constitutes a part of the housing thereof. Although attachment thereof thereto may be accomplished by any suitable means--4 as, for example, by welding, bolting, or the like-it should preferably be done in such manner that would facilitate assembly and disassembly and still be liquid tight.

Rear wall 24 contains an aperture 25 of such compatible configuration as to accommodate the insertion of an assembled transducer array 26, including a flanged frame 27, within which array 26 is mounted to form a unitary component or module that is readily changed or replaced. Said transducer array, of course, contains a predetermined plurality of piezoelectric energy converter elements 28. They may be made, for example, of lead zirconate or barium titanate crystals or Clevite PZT-S crystals, manufactured by the Clevite Corporation of Cleveland, Ohio, and they are preferably mounted in said frame 27 by means of an appropriate epoxy potting material 29. For this purpose, Scotchcast No. 8, manufactured by Minnesota Mining and Manufacturing Company, has been found to be satisfactory.

Frame 27, and thus transducer array 26, is securely mounted within aperture by means of screws 30 which extend through countersunk holes 31 in the ange portion thereof and into compatible threaded holes 32 located in rear wall 24.

Disposed around the remaining 240 of the diametrical periphery of top and bottom plates 22 and 23 is a curved acoustic window 33. This window is preferably made of Absonic-A (acrylonitrile butadine styrene), manufactured by the B. F. Goodrich Company; however, in the event a different window index of refraction is necessary for any particular operational situation, any suitable substantially acoustically clear material having the required thickness and index of refraction may be substituted therefor, inasmuch as so doing would be well within the purview of one skilled in the art having the benet of the teachings herewith presented. For example, a material may be selected for window 33 which has such acoustic index of refraction that a predetermined critical angle of reflection of each of the acoustical energy waveforms incident thereon occurs thereat. For a given transducer or window diameter, this causes a given diameter aperture to be effected therein which, in turn, causes the exclusion of all of said incident acoustical energy waveforms from the lens of the subject transducer that would not be properly focused at or substantially at one or more (as the case may be) of the aforementioned piezoelectric energy converters 28 of transducer -array 26. Of course, due to the existence of the aforesaid critical angle of reflection, the aperture effected thereby is dynamic, in that it will automatically move to be effectively centered on each of the incoming acoustical wavefronts, regardless of the direction within a given field of view from which they come. Obviously, for a given index of refraction, if the diameter of the lens (or the radius of curvature of the window) is changed, the effective diameter of the aforementioned acoustical aperture is changed proportionally.

The aforementioned selection of window thickness is contingent upon the operational frequency selected. Thus, if a preferred operational frequency of 125 kilocycles per second is selected as being optimum for some particular operational circumstances, it has been found that an Absonic-A window thickness of i752 inch is quite satisfactory. Accordingly, general design of the acoustical window thickness may be defined by C=rf (1) where C is a constant having a value of the order of 11.72, t is window thickness in inches, and f is operational frequency in kilocycles per second.

A pair of angle-shaped mounting rings 34 and 35, each of which has a plurality of holes 36 and 37, is mounted around the outer edges of top and bottom plates 22 and 23 and window 33 in such manner as to hold window 34 in place thereagainst, respectively. A like plurality of screws 38 and 39 extend through said ring holes and into compatible threaded holes disposed around the outside diameter of top and bottom plates 22 and 23 for the positive securing thereof.

Obviously, each of the aforementioned energy converter elements has a pair of electrical lead wires 40 connected thereto which extend preferably as an electrical cable 41 along the underside of top plate 22, through a hole 42 and through any suitable seal, such as, for instance, a Kovar glass bead-metal seal 43, manufactured by the Kovar Manufacturing Company, which is mounted on top of said plate 22, as by welding or the like.

As shown in FIG. 3, seal 43 includes a metal plate 44, which is the part that is welded or otherwise tightly secured to upper plate 22 over hole 42. Extending through at least one hole in plate 44 0f seal 43 iS a glass bead 45;

which, in turn, acts as a seal forsaid electrical cable 41 (containing electrical lead wires 40) as it passes therethrough. Of course, if so desired, each of lead wires 40 may pass through seal 43 by means of its own individual glass sealing bead. But for the purpose of simplifying the disclosure thereof in FIG. 3, only one glass bead is used. Of course, when said seal 43 is in place, a sealed chamber 47 is effected inside housing 21.

On substantially the entire inner surface of chamber 47, with the exception of that part formed by acoustic window 33 and that part occupied by transducer array 26, a sound absorbing material or anechoic lining 48, such as, for example, a butal type rubber impregnated with aluminum particles of suitable density or Soab, manufactured by the B. F. Goodrich Company, is mounted as by glueing or any other suitable means. A satisfactory adhesive 49 (shown in FIG. 5 only) for this purpose is Vulcalock, manufactured by the B. F. Goodrich Company.

As may best be seen in FIGS. 3, 5, and 6, the aforementioned anechoic lining 48 preferably has a waffle iron type of geometrical configuration. This particular pattern or configuration has been found to have excellent sound absorbing properties in this particular structural arrangement, since, as compared to relatively smooth sound absorbing linings, it increases the sound absorbing area to a considerable extent.

Disposed at any suitable location on upper plate 22 is a tapped and threaded lluid lter hole 50, in which is screwed a threaded plug 51. Through the hole, a uid 52 of predetermined index of refraction is poured, until it entirely iills chamber 47. Then, of course, plug 51 is inserted to maintain said uid therein. In this particular preferred embodiment, refracting fluid 52 consists of a predetermined mixture of two parts of grade FS-S Fluorolube, (CFZCFCl)x with molecular weight 560, (manufactured by Minnesota Mining and Manufacturing Cornpany of St. Paul, Minnesota) and one part Dichlorooctafluoro butane, (C4Cl2F8), (manufactured by Hooker Chemical Company of Niagara Falls, N.Y.), by volume.

A superstructure 53 for the mounting of electronic and electrical apparatus (not shown) for driving or responding to the subject transducer is securely attached to upper plate 22 at such location as to receive and enclose the aforementioned wires and glass beaded metal seal 43 and in such manner as to insulate them from the transducers ambient environment. In this particular instance, superstructure 53 is made in two parts, a lower cylinder 54 having an upper ange 55 containing bolt holes 56, and an upper cylinder 57 having a closed top 58 and a lower ange 59, likewise containing bolt holes 61 which are compatible with bolt holes 56. A resilient gasket 62 for sealing the two sections of superstructure 53 is inserted between the aforementioned compatible flanges 55 and 59, and, of course, it, too, has bolt holes (not seen) which are in line with bolt holes 56 and 61. A plurality of bolts 63 are respectively inserted in said bolt holes and are tightened by nuts 64 in such manner and with such amount of torque as to make superstructure 53 waterproof.

A pipe nipple 65 may be welded in a hole 66 of a boss 67 on superstructure 53 as the outlet for the electrical conductors, and a hose 68, connected thereto by means of a clamp 69, protects the electrical conductors contained therein as they traverse the distance to the utilization apparatus.

A support ring 71 having holes 72 disposed therein is securely attached to upper plate 22, as by welding or the like. Cables 73 are attached to said support ring in such manner as to act as a sling therefor, so as to hold the entire transducer at a desired level and attitude in the water 0r other operation medium. Another ring 74 is likewise welded to bottom plate 23 for supporting the entire transducer on its bottom side, if so desired. Of course, it should be understood that any other appropriate support means may be used with the subject transducer withu out violating the scope of this invention, since so doing would merely be a matter of design choice of the artisan.

Because it is not always possible to control the fit of all of parts included in the aforementioned housing assembly, if so desired, any suitable adhesive or sealing material 75 may be inserted between all joining or abutting surfaces thereof in such manner as to effect a uid and/or watertight, sealed housing. Thus, the water of the ambient environ-ment will remain outside thereof and the refracting fluid will remain inside thereof without leakage, regardless of the water depth lat which the transducer is deployed during operations or the internal pressure of said refractive fluid contained therein.

The general theory of operation will now be discussed as follows in conjunction with FIG. 7, a diagram representing a general focusing system. Letting 2 be the surface boundary of 'an arbitrary closed volume filled with a material having an acoustic index of refraction n, then where cris the sound velocity in the medium exterior to the volume, and v is the sound velocity within said volume.

There are two conditions which must 'be satisfied if plane waves arriving at a given plane 1r are to be focused at point F. First, all the energy arriving at F from the plane 1r must be in phase. This implies equal acoustic path lengths between F and every point on plane 1r, i.e.,

PQ+17QF=CONSTANT (3) where Q is dened as the point where the incoming acoustical energy enters the aforementioned volume. Second, the index of refraction n must be chosen so that all rays incident on Surface E are refracted as nearly as possible toward point F. That is, i7 must be chosen so that the angle rp between the acoustical ray path after it has been refracted within the medium of said volume and the most direct path which it could take to reach point F after entering said volume will equal zero. No simple acoustic lens will satisfy these two focusing conditions simultaneously over a useful range of angles 6 located between the path of an incoming acoustical ray and an imaginary line normal to the tangent of surface 2 at point Q, the acoustical ray entry point. Two lens types for which the focusing conditions described above are approximately true are the cylindrical lens and the elliptical lens.

A unique feature of the cylindrical lens is the ability to form a large number of simultaneous and independent directive beams over a 360 degree eld of View. This characteristic is a result of the rotational symmetry which exists about the axis of the lens.

The elliptical lens will not have this rotational symmetry so that a 360 degree eld of view will not be possible. However, in the elliptical lens, the aforesaid focusing conditions -will be more accurately satisfied and, accordingly, narrower beamwidths will be achieved thereby. Hence, improved resolving power of the lens will be obtained for a limited eld of view.

The theory of a cylindrical lens will now be discussed in detail. In this discussion, and with respect to all the equations that follow, all of the units are in seconds for time, meters for distance, and degrees for angles, unless it would be obvious to the artisan that they should be otherwise.

In addition to the focusing conditions stated above, an additional condition was imposed, namely, that the focal point occur within the cylinder. This was required to maintain rotational symmetry and to facilitate actual construction. This requirement is a constraint on the aforementioned second focusing condition which states that 1; be chosen such that all incident rays are refracted as nearly as possible toward point F. From the exemplary ray path diagram shown in FIG. 8, it may be observed that for an index of refraction of approximately 1.9 the second focusing condition is most nearly satisfied. For

, 6 this value of 11, the focal position of all incoming acoustical rays is near the back surface of the lens.

The fact that all of said rays intersect in a region at the back surface of the lens does not imply that the energy arrives in phase in this region. To see how well the contributions from various incident angles agree in phase at the back surface of the lens, a plot of the relative path error as a function of incident angle may be made.

From FIG. 9, it may be observed that the path length for a ray at incident angle 0 is given by where PL is the path length PQF,

R is the radius of the lens, and

0 and v7 are the equivalent of the 0 and v7 defined previously.

If the acoustical ray path having a zero incidence angle 0 is taken as a reference path, then the relative path error is When the relative path error is plotted as a function of 0 for several values of 1 it may readily be seen that the relative path error is smallest [except at large angles 9] for an index of refraction of the medium of approximately 1.9. Before deciding on the precise value of n to use in an acoustic lens, it is necessary to consider the effects of the shell which must be used to enclose the fluid.

For materials which possess the mechanical characteristics necessary for construction of the lens, a critical incident angle 0c exists at the water shell interface. This critical angle is defined as being equal to arcsin 17. The effect of this critical angle is to place an upper limit on the incident angle at which rays may enter the lens. Of course, these high upper limit incident angle rays are of little use, since their large relative path errors will introduce spherical aberrations andvdegrade the performance of the lens. Suppressing these rays will improve the focusing characteristics of the lens. This is analogous to the aperture stop in opticsrwhich reduces spherical aberrations by blocking rays incident at large angles. In the acoustic lens constiuting this invention, the aperturing caused by the critical angle 0c is a desirable effect, and proper control of this factor has contributed to the success thereof. It has been found that when a shell material having a critical angle 9c near 60 degrees and a fluid having an index of refraction near 1.9 is used, relative path errors of less than .2% will result. It is significant that the effective aperture length is reduced by only 15%, when a critical angle of 60 ydegrees is used.

It has been shown that neither of the two aforementioned focusing conditions are exactly satisfied lby the cylindrical lens. However, it has also been found that said two focusing conditions can be approximately satised while maintaining rotational symmetry of the lens and the resultant 360 degree field of view effected thereby. To do so, of course, it is necessary to further define the effects of these approximations on the formation of the focal region within the lens, and to accomplish this, requires a more fundamental analysis than simple ray theory' concepts allow. A

The amplitude and phase distribution on the back surface of a cylindrical liquid lens is determined by the vector sum of the individual contributions from every point on a plane wave front incident on the principal plane of the lens. To perform this vector sum, the amplitude and phase contributions from each point on the principal plane must be determined. The amplitude may be determined by taking into account the following effects:

(1) Transmission losses due to the impedance mismatch at the water-window interface;

cal aberrations is imperative if beaniwidths less than 6 are required.

Employment of the lens window in this manner yields dynamic aperturing for all incident sound waves, regardless of the relative angular position of the sound source.

(2) Transmission losses due to the impedance mis- 5 match at the window-liquid interface inside the lens', Thls 1S of tal Importance smc@ the lens accomphshes (3) Effects of acoustical energy spreading within the the Same-aperturmg erecffor a.sound source located it lens; and any relative anguljtr position with respect thereto as is (4) Effects of Obliquity of the acoustical energy from acomphshed at For .this .reason a Wide .field of V1e.w the an g1 e (i) Shown in FIG la lo withno degradation of directional response is effected in the instant invention. Let the function Q(x,0) include these effects. This func- With the parameters deiied as in FIG. 10, the terms tion represents the amplitude at the point x on the back incorporated in the aforementionad Equation 7 may be surface of the lens contributed by the energy which enters expressed as follows; the lens at an angle 0. 15 1 The phase of the contribution at the point x on the Q(x,0)=-|:1+ back surface of the lens is determined by the total acoustic 2 path length P(x,0) which is composed of individual path R i sin 0 lengths im (TTf/ 1 (T i.

(1) The acoustic path length from the principal plane 20 i to the lens window; tazrl xlllmhh) :I

(2) The acoustic path length through the window; and x/RZ-l-xZ-I-siii (alpha.)

(3) The acoustic path length through the refracting (10) medium within the lens to the point x.

The phase at the point x of the contribution incident at f R 9 angle 0 may then be written as ejkP(x,i9), Where j is equal 2o P(x1)=R l-COS 0) "i-nSR Sm [sm-1( t'sl; to l, and k is 21r/A. ein 0 The vector summation of the contributions with incisin*1 /siii 0+ dent angles less than @c may be represented by a complex s function A(x). :This function will describe the resultant 30 nfM R sin (alpha)]z+[w/m i R sin (mphwp phase and amplitude distribution on the back `surface of the lens and may be written (n) where the variables distances x and angles 0 are defined R m R ns in FIG. 10. A(x) is the impulse response of the lens, and sin 0 W 0c is the critical angle of the lens window material. This tan-1|:- `/1-( il angle is defined as ns ne 0c=arc sin 17s (8) 40 (12) where 17s is the refractive index of the window material. Where Q(x,0) is the amplitude contributed at x by energy enternf is the ndex of refractlfm of the uld ing the lens at angle 0, and kP(x,0) is the phase of this ns ,1S the mdfx of refraction of the Shen contribution. If it is desired to block acoustical rays inci- 45 R 1S the rdlus of the lens dent on the lens at angles greater than 45, the operative t 1 5 the thlciiess of the Shen angles of the pret-cned embodiment of the Subject invem x is the position of an arbitrary point on the back surface tion herewith disclosed, the index of refraction of the winof the lens dow material must Ibe selected in accordance with alpha 1S a.dumr.ny.var.lable and 0 is a variable indicating angle of incidence of the energy Ww: Sm 9c (9) 50 entering the lens.

In such case, where 9c is selected to be 45, 17W obviously The fulctlOns Q03@ and P) are dependent 0H becomes 707. the lens diameter and window thickness, the acoustic im- For operation in sca water, c, the velocity of sound in Pedance PY (that is: the density 0f iii@ material in' water, is approximately 1500 meters per second. The 55 V01Ved- P Unies the Speefi 0f Sound if 1I1 it.) and the velocity of sound in the Window material then becomes refractive indices of the window and Huid within the lens.

Therefore, effects of all these lens parameters may be :@Ltglo() meters obtained by evaluating the aforesaid integral of Equation .707 7. The results of the numerical integration thereof, as

per second when the aforesaid 45 incident angle is used 60 determined by actual Computation thereof by digital COUP as the design criterion. puter, constitute the directional response of the lens to Since the sound velocity in the window must be 2100 a plane Wave. Theoretical directivity patterns have been meters per second in order to meet the 45 critical incicomputed over a wide range of lens parameters. The dent angle requirement indicated above, the material specific values chosen for these parameters for this particthereof must be selected to have such internal sound f ular invention were those of commercially available matevelocity characteristics. In this particular instance, Ab- 6 rials which possess desirable mechanical and acoustic sonic-A has been found to be eminently satisfactory. properties.

Using this criterion to choose the lens shell material will The following table lists the material characteristics suppress spherical aberrations and permit the subject lens used in computing the directivity patterns of this invento achieve beamwidths less than 1. Reduction of spherition:

Acoustic Index Impedance, of

Function Material gm./cin.2see. refraction Lens Shell Absoiiic-A 2. 27)(105 0.7

Refractive Fluid Dicliloioocta fluorobutauc arid ilourolubc in ratio 1:2 l. L10X10 5 1. 92

When using the aforesaid materials, it has been found that excellent directivity patterns are obtained.

Many operating conditions require that the lens be focused at some predetermined distance. When the lens is filled with a fluid having an index of refraction of 1.92, any object between ranges of D20( and infinity are in focus, where D is the diameter of the transducer and A is the wavelength at which it is operated. To focus the lens on an object at a distance less than D2/)\ it is necessary to adjust the refractive index nf of the lens fluid so that the following relationship will be satisfied.

where D is the lens diameter in meters, R is the object distance in meters7 and 0c is as previously defined.

In FIG. 11, there is shown a comparison of the relative response of the main lobe and first side lobe of the lens as a function of d/)\, where d represents the diameter of the lens and A represents the wavelength at which it is being operated. In this figure there is also shown a comparison of the performance of a lens containing a window having the same acoustic properties as the surrounding medium with a lens containing an Absonic-A Window of 0.139 cm. thickness. This figure shows that with an Absonic-A window higher main lobe response can be achieved with greater side lobe suppression over a wider range of d/x values. To understand the effects of these two windows, it should be recalled that sound waves incident at large angles are blocked if a critical angle exists. For a window with the same acoustic properties as the surrounding medium, no critical angle exists and all incident sound waves will enter the lens. However, those sound waves entering the lens at high incidence angles introduce large phase errors at the focal point and cause Spherical aberrations.

One additional result of blocking the higher incident angle acoustical rays is to reduce the effective aperture of the lens. This is illustrated in FIG. 12, which compares the 3 db beam widths of the lens as a function of d/7( for the two types of shells discussed above. Since the effective aperture is reduced by the Absonic-A window, the beam width at a given d/ ratio is larger than that obtained with a window having the same acoustic properties as the surrounding medium. However, the reduction in spherical aberrations which the Absonic-A effects allows a lens with a given diameter to be operated at a higher d/A ratio, i.e., at a higher frequency.

For instance, consider a lens whose diameter is 40 centimeters, and furthermore, require that the side lobes thereof be at least l db below the main lobe. If the lens has a window with acoustic properties similar to the surrounding medium, the maximum value of d/k is limited to 85 as indicated in FIG. 11. This corresponds to a maximum operating frequency of 320 kilocycles per second and a minimum beam with of 0.8" as indicated in FIG. 12. If the lens has an Absonic-A window 0.317 cm. thick, the maximum value of d/ A is extended to 160. When this lens is operated at 320 kilocycles per second, the beam width is 1.0; however, due to the reduction of spherical aberrations, the lens may be operated at 475 kilocycles per second, resulting in a beam width of approximately 0.50 while maintaining a 10 db side lobe level. Thus, improved directional characteristics can be realized by aperturing the lens to suppress spherical aberrations.

The results of the foregoing theoretical discussion of cylindrical acoustic lenses may be summarized as follows:

(l) As a result of the failure to satisfy the first focusing condition, relative path errors occur. These errors may be interpreted as spherical aberrations;

(2) In optical lenses, spherical aberrations may be reduced by aperturing. In acoustic lenses, spherical aberrations may be reduced in an analogous manner by choosing a window material which has an index of refraction less than that of the surrounding medium. This results in a critical angle which will limit the maximum angle at which acoustical rays may enter the lens; and

(3) Theoretically, any beam width can be achieved if the leus is apertured to reduce spherical aberrations suf ficiently, and if a large enough effective aperture is preserved. However, for beam widths less than about 0.5 the lens diameter may become impractical for some applications.

A possible alternate embodiment of the subject invention-namely, elliptically configured lens-is also possible and is, of course, intended to be included within the scope of this invention. The theory thereof will now be discussed briefly and compared with that of the previously discussed cylindrical lens.

To obtain beam widths less than 0.5 with an acoustic lens, some technique other than aperturing is preferred. One possible method of accomplishing this is to shape the lens surface so that the aforementioned lirst focusing condition will be exactly satisfied. The lens surface will satisfy this condition if equal acoustic path lengths exist between the focal point F and every point on the incident plane wave P in FIG. 13.

Considering now FIG. 13, the parameters incorporated therein are herewith defined as follows for purpose of clarity:

P is a plane tangent to surface E at point A, X is a perpendicular distance from plane P to an arbid is the distance from point A to focal point F Within the fluid volume enclosed by surface E,

Y is perpendicular distance from line AF to point Q, n is the index of refraction of the fluid volume enclosed by surface E.

Further considering FIG. 13, it may now be determined that to satisfy the aforesaid equal acoustic path length requirement, it is necessary that the conditions Of the following Equation 14 be met:

xl11((dx)2lY2)1/2=11d (14) Solving for y2 this becomes 1-112 M11-1)] fm2( 2 )Jfzi'T t 15) Rewriting,

HdM-1) 2 t/ 2pf [nd(11-1) 2 [w+ 1*?)2 712 1-17 (16) Translating the original of coordinates to Wdh-1) o] (1-112) (17) and dividing through by the constant on the Tight,

x2 y2 [ndo-uil* loro 2 1 1*112 (aL-1)2 (18) which is recognized as the standard form of the ellipse where (19) is the standard mathematical equation for an ellipse, and where Thus therlens surface which exactly satisfies the rst focusing condition is an ellipse. It'is of interest to' determine the eccentricity of the ellipse. Eccentricity, e, is defined as e=c/a (22 where where a and b are as above and c is a dummy variable. Identifying a and b in Equation 23 and solving for c allows Equation 24 to be written as Thus, by choosing the lens surface to be an ellipse and using a liquid with an index of refraction equal to the reciprocal of the eccentricity, the first focusing condition is exactly satisfied. This condition can be satisfied for any index of refraction; however, the value of v7 must be chosen so that the second focusing condition is also satisfied. It has been found that the optimum index of refraction is near 2. This will give a minor to major axis ratio of 0.866, so that the shape of an elliptical lens will depart only slightly from that of a cylinder.

The actual operation of the subject invention corresponds very well with the aforesaid design theory therefor.

ln actual operation, the subject acoustical lens transducer is reversible, in that it may be used as either a receiving transducer or a transmitting trans-ducer, as desired. In either case, the response and radiation directivity patterns will be substantially identical and are typically depicted in FIG. 14.

When used to image underwater target objects, the subject transducer is deployed within the water with the window thereof pointed in the general direction of the area being searched. Of course, as previously intimated, it may be used as either the receiving transducer of a receiving sonar system only or as the transmitreceive transducer of a transmitting and receiving sonar system, or both, as desired.

For the sake of simplifying this disclosure, it will be assumed that it is being used as a receiving transducer for a sonar receiver for receiving acoustical echo signals from a subaqueous target.

Due to the wide field of View, the target search sector is considerable, and thus a target object may be easily acquired and focused on the energy converting sensors located at the rear of the transducer, even though it lies somewhat off the optical axis thereof. The aforesaid focusing of the acquired target image is, of course, facilitated and greatly improved from a fidelity standpoint, due to the aforementioned unique liquid refraction and structural tuning of the acoustical lens portion thereof.

In view of the foregoing, it may readily be seen that the unusual interaction of the associated elements of the unique combination of elements constituting this invention produces operational target imaging results that are superior to those heretofore obtained from comparable devices of the prior art. This, in turn, facilitates the finding and identifying of numerous types of underwater target objects, which obviously improves the entire sonar operations in general.

The structural configuration of the preferred embodiment herewith disclosed is that of a cylindrical type transducer. However, as previously mentioned, it should be understood that an elliptical configuration, a hemispherical configuration, a truncated spherical configuration, or a spherical configuration may be effected merely by designing the upper and lower plates accordingly. In any case, so doing would be Well within the purview of the skilled artisan having the benefit of the teachings herewith presented, and, obviously, so doing would not violate the spirit and scope of this invention.

In view of the foregoing, it should also be obvious that many other modifications and embodiments of the subject invention will readily come ,to the mind of one skilled in the art'having the benefit of the teachings presented in the foregoing description and the drawings. It is, therefore, to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A reversible transducer for converting acoustical energy received from within subaqueous medium into electrical energy proportional thereto and for converting received electrical energy into acoustical energy proportional thereto, comprising in combination:

a watertight housing having a plurality of walls and a predetermined geometrical configuration;

an acoustical window means, mounted in one of the walls of said housing, having an acoustic index of refraction for effecting a predetermined critical angle of total reflection to incoming acoustical energy wavefronts incident thereon for producing a givenY diameter aperture therein that automatically relocates in such manner as to be substantially centered on all of said incoming acoustical energy wavefronts, respectively, regardless of the direction within a given field of view `from which they arrive and for controlling third and fifth order aberrations therein;

an array of energy converters mounted in another wall of said housing in opposite disposition with the aforesaid acoustical window means;

means extending through said watertight housing and connected to each energy converter of said array of energy converters for timely conduction of electrical energy therefrom to a utilization apparatus and thereto from a utilization apparatus;

an anechoic lining means mounted on substantially all of the inside surfaces of said watertight housing not occupied by said array of energy converters and said acoustical window means; and

a liquid means, having an acoustical index of refraction for effecting the focusing of acoustical energy received through said acoustical window means on said array of energy converters, disposed in said watertight housing in such manner as to effect the complete filling thereof therewith and, therefore, be in contact with the inner surfaces of said anechoic lining means, the inside of said acoustical window means, and the energy converters of the aforesaid array of energy converters.

2. The device of claim 1 wherein the plurality of walls of said watertight housing are of aluminum.

3. The device of claim 1 wherein said acoustical window means mounted in one of the walls of said housing is of acrylonitrile butadine styrene having a predetermined thickness.

4. The device of claim 1 wherein the energy converters of said array of said energy converter mounted in another wall of said housing in opposite disposition with the aforesaid acoustical window means are composed of lead zirconate.

5. The device of claim 1 wherein said anechoic lining means mounted on substantially all of the inside surfaces of said watertight housing not occupied by said array of energy converters and said acoustical window means consists of a predetermined thickness of butyl rubber impregnated with a predetermined density of aluminum particles.

6. The device of claim 1 wherein said liquid means, having an acoustical index of refraction for effecting the focusing of acoustical energy received through said acoustical window means on said array of energy converters, disposed in said watertight housing in such manner as to effect the complete filling thereof therewith and, therefore, be in contact with the inner surfaces of said anechoic lining means, the inside of said acoustical window means, and the energy converters of the aforesaid array of energy converters comprises two parts by volume of (CFgCFCl)x with predetermined molecular weight and one part C4Cl2F8.

7. The device of claim 1 wherein said liquid means, having an acoustical index of refraction for effecting the focus of acoustical energy received through said acoustical window means on said array of energy converters, disposed in said watertight housing in such manner as to effect the complete iilling thereof therewith and, therefore, be in contact with the inner surfaces of said anechoic lining means, the inside of said acoustical window means, and the energy converters of the aforesaid array of energy converters comprises a mixture of iluids having such predetermined ratios by volume that said transducer is focused on objects at distances less than D2/ where D is the diameter of the transducer in meters and A is the wavelength at which it separates in meters.

8. The device of claim 1 wherein the geometrical configuration of said watertight housing is a cylindrical configuration.

9. The invention of claim 1 further characterized by means connected to said watertight housing for locating said entire reversible transducer in predetermined disposition within a subaqueous medium.

10. The invention of claim 1 further characterized by means mounted on said watertight housing for encasing a portion of said utilization apparatus connected to the aforesaid electrical energy conduction means.

11. A reversible transducer for converting acoustical energy received from within a subaqueous medium into electrical energy proportional thereto and for converting received electrical energy into acoustical energy proportional thereto, comprising in combination:

a top plate having a predetermined thickness and a circular configuration;

a bottom plate, having a thickness and a circular configuration substantially similar to said top plate, spatially disposed a predetermined distance below said top plate;

a curved rear wall disposed around and attached to predetermined similar rear peripheries of said top and bottom plates in such manner as to form a fluid tight seal therebetween;

a curved front acoustical window mounted around and attached to the peripheries of said top and bottom plates not occupied by the aforesaid curved rear wall, with the ends thereof butted against the ends of said curved rear wall respectively, in such manner as to form a uid tight seal therebetween;

an aperture located in said curved rear wall;

a frame, having a curvature compatible with that of said rear wall, securely mounted in the aperture of said rear wall in a watertight configuration;

a plurality of electroacoustical energy converters mounted in said curved frame in such manner that the active faces thereof are contiguously disposed with each other on the focal area of the aforesaid reversible transducer;

an anechoic lining, having a waffle iron inside surface configuration, mounted on substantially the entire inner surface of said top plate, Ibottom plate, and rear wall not occupied by said plurality of electroacoustical energy converters; and

a liquid, having such an acoustical index of refraction as to effect focusing of acoustical energy directionally received through said curved front acoustical window on respective active faces of said plurality of electroacoustical energy converters, disposed wthin the chamber formed by the aforesaid interconnected top plate, bottom plate, rear wall, and front window.

References Cited UNITED STATES PATENTS 2,420,676 5/ 1947 Peterson 340-8 2,436,377 2/ 1948 Briggs et al 340-8 2,803,128 8/1957 Petermann.

2,994,400 8/ 1961 Heller 340-5 RODNEY D. BENNETT, JR., Primary Examiner BRIAN L. RIBANDO, Assistant Examiner 

